[mlir] Unroll-and-jam loops with iter_args.

Unroll-and-jam currently doesn't work when the loop being unroll-and-jammed
or any of its inner loops has iter_args. This patch modifies the
unroll-and-jam utility to support loops with iter_args.

Reviewed By: bondhugula

Differential Revision: https://reviews.llvm.org/D110085